296 EEPORT ON NATIONAL MUSEUM, 1886. 



white variety is a silicate of alumiua aud potash with small amounts of 

 irou, soda, maguesia, aud water. 



The kind, amount, and disposition of mica in a building stone has a 

 very important bearing upon its working and weathering qualities as 

 well as general fitness for architectural purposes. If it occurs in any 

 abundance and the folia are arranged in parallel layers the rock splits 

 much more readily in a direction parallel to the mica laminse than in 

 that at right angles to them. Mica is itself moreover " soft and fissile, 

 and hence is an element of weakness." It also receives a polish only 

 with difficulty and which is soon lost upon exposure to the weather. 

 Black mica, moreover, owing to its large percentage of iron, is liable to 

 succumb to atmospheric agwicies.* 



The finest grades of building stone should contain mica oidy in 

 SMall flakes, and these evenly distributed throughout the mass of the 

 rock. 



From the marked contrast in color of the two micas it follows that 

 they have a decided influence upon the color of the rock containing 

 them. Folia of black mica in any abundance naturally give the rock 

 a dark-gray hue, while the white mica, being nearly colorless, has a 

 neutral effect. Hence, other things being equal, muscovite granites 

 are much lighter in color than those in which biotite is the character- 

 izing accessory. 



Other micas common in such stone as are used for building are lepi- 

 domelane and i)hlogopite. The first of these is black in color and 

 closely resembles biotite, from which, however, it diflers in containing 

 smaller proportions of the protoxide of iron and in the folia being opaque 

 and inelastic. For all practical purposes this mica is, however, identical 

 with biotite, and no distinction has been attempted in the present work. 

 Phlogopite is colorless like muscovite, from which it can often be distin- 

 guished only with difficulty. It is a common constituent of many lime- 

 stones, dolomites, and serjientinous rocks. 



AMPHIBOLE. Hornblende. Hanlnoss, 5 to 6. 



Two i)rincipal varieties of this mineral are recognized: (1) The non- 

 aluminous, including the white, gray, and pale green, often fibrous 

 forms as tremolite, actinolite and asbestus, and (2) the aluminous,which 

 includes the dark-green, brown, and black varieties. The aluminous 

 variety, common hornblende, is an original and essential constituent of 

 diorite, and of many varieties of granite, gneiss, syenite, schist, ande- 

 site and trachyte, and is also jiresent as a secondary constituent in 

 many rocks, resulting from the molecular alteration of the augite. The 



* Dr. P. Schweitzer while studying the superficial decomposition of the gneiss of New 

 York Island, discovered that the hlack mica, after getting first coated with a brown 

 film of oxide of iron, " rai)idly disintegrated and disappeared," while the white 

 mica iiossessing greater powers of endurance remains fresh aud intact. — Chem. News, 

 IV, 1874, p. 444. 



The same phenomena may be noticed in the mica schists about Washington, D. C. 



